1. Field of the Invention
This invention relates to a pigtail fiber module to be used mainly in fiber lasers, the pigtail fiber module including a large mode area fiber and a fiber holding member provided at a tip end of the fiber. Particularly, this invention relates to an improvement in a pigtail fiber module which emits beams having a sectional shape resembling a perfect circle as close as possible.
2. Description of the Prior Art
A pigtail fiber module is a type of light transmission media used in optical communications, laser processing machines, and so forth. Recently, the power of beams transmitted in laser processing machines and so forth has been increased, and also a wide variety of optical fibers have been in use. In addition, a current mainstream optical fiber usable for high power output is a large mode area fiber (hereinafter also referred to as “LMA fiber” in some cases).
As a pigtail fiber module using this type of large mode area fiber, there has been known a conventional technique (pigtail module) described in Japanese Patent Application Publication No. 2012-255999 (Patent Document 1).
The pigtail module according to the conventional technique is characterized as follows. Specifically, the pigtail module includes: a fiber holding member; and a pigtail fiber (large mode area fiber) having a fiber core wire composed of a core where single-mode light travels and a clad surrounding an outer periphery of the core, and a protective coating surrounding an outer periphery of the fiber core wire. The fiber core wire is exposed by removing a portion of the protective coating, and fixed in a through hole of the fiber holding member with an adhesive made of a thermosetting resin (thermosetting adhesive). Moreover, the through hole is set to have an inner diameter satisfying such a condition that an average gap between the fiber core wire (clad) and a portion in the through hole where the fiber core wire (clad) is fixed with the adhesive [(the inner diameter of the through hole−the diameter of the clad)/2] is less than 1 μm (see claim 1 of Patent Document 1).
Moreover, by setting the through hole to have an inner diameter satisfying the condition that the average gap [(the inner diameter of the through hole−the diameter of the clad)/2] is less than 1 μm, the adhesive present in the gap between the fiber core wire (clad) and the through hole also has a thickness of less than 1 μm. This decreases a stress on the optical fiber due to the curing and contraction of the adhesive. Hence, in the pigtail module according to the conventional technique, the shape of a light intensity distribution of emitted beams can supposedly be kept in the same state as when the beams enter the pigtail module (paragraphs 0011 and 0018 of see Patent Document 1).
Meanwhile, after conducting a technical evaluation on the pigtail module according to the conventional technique, the present inventor has noticed the following problems on the contrary to the description of Patent Document 1.
Specifically, in this type of large mode area fiber, an end cap is fused to a tip end of the fiber (such as coreless fiber), and a variation is observed in the light intensity distribution of beams emitted from an end surface of the end cap. Moreover, when the large mode area fiber is used in a laser processing machine, since such a variation in the light intensity distribution greatly influences the processing performance and processing precision, it is desirable that the fiber should keep the original light intensity distribution.
Hence, the present inventor has examined the variation in the light intensity distribution according to the conventional technique in detail. The result has revealed that a cause of the variation is an adhesive made of a thermosetting resin (thermosetting adhesive) used for the adhesion of the fiber core wire (clad) to the through hole of the fiber holding member. To be more specific, although the adhesive present in the gap between the fiber core wire (clad) and the through hole has a thickness of less than 1 μm, an external stress is still applied on the fiber core wire (clad) by the curing and contraction of the thermosetting adhesive, causing a variation in the light intensity distribution. Furthermore, the variation is also caused by the fact that, in the pigtail module according to Patent Document 1, the setting to satisfy the condition that the average gap [(the inner diameter of the through hole−the diameter of the clad)/2] is less than 1 μm is difficult in reality in consideration of the processing precision of the fiber holding member and the large mode area fiber (LMA fiber).